REVERSIBILITY OF MORPHOGENESIS 13 



rearrangement of the cells to form the beginnings of normal sponge 

 structure, it has not yet been proven that all the dedifferentiated 

 cells are equivalent in their potentialities and functions and 

 actually can assume any of the roles played by the different 

 types of sponge cell. 



In view of this status of the question the facts described for 

 Bursaria may have an added interest in so far as they fulfil the 

 necessary conditions for experimental proof that cell differentia- 

 tion is in this case a reversible process. 3 



All that is implied in the expression reversibility of differ- 

 entiation is that the parts or materials of a differentiated system 

 when separated, somehow either reform a structure which is made 

 up fundamentally of the same or duplicate elementary parts, 

 but not necessarily having the same spacial relations to one an- 

 other which they had in the original sytem, or are reformed by 

 the same steps but in the reverse order. What evidence can 



3 It is important for what precedes and follows to make clear the meaning 

 of the term reversibility. Let us represent a specific configuration of material 



parts by abcde . A specific kind of protein molecule might be used as an 



illustration. The elementary parts represented by a, b, c, d, e would be the dif- 

 ferent amino-acids of which the specific molecule was formed. 



We will assume that the reaction abcde , — > a, b, c, d, e - -, represents a 



dedifferentiation of the system abcde , similar to the breaking down of a 



specific protein molecule into its constituent amino-acid, groups. The reverse 

 process, a, b, c, d, e, — > abcde , would represent differentiation, a proc- 

 ess of reformation of the system or synthesis. If now the reaction abcde , 



— > a, b, c, d, e , represented only the beginning and end results of a process 



which as a matter of fact took place in steps having a definite sequence (con- 

 secutive reactions), perfect reversibility would be represented by a reversed 

 sequence of these steps or consecutive reactions. On the other hand if a, b, c 

 d, e, , united to form abcde , but the sequence of the steps in this syn- 

 thesis were not the reverse of the sequence of the steps for abcde , — > a, b, 



c, d, e, , then the processs would not be strictly reversible so far as the 



method of synthesis was concerned but would be considered reversible in so far 



as the end results were the same. Furthermore if abcde , separated to 



form a, b, c, d, e, and these elements reunited to form debca , then the proc- 

 ess would be considered still less perfect in its reversibility. However, the 

 process could be said to be reversible in so far as it led to the separation and 

 reformation of complexes which had the same elementary percentage composition. 



Chemically speaking abcde, , and debca , would be isomers. All these 



degrees of reversibility are exemplified in various syntheses by enzymes. See 

 Euler, General Chemistry of the Enzymes, page 261. 



